1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transflective type LCD device which can maximize the light-use efficiency in a transmitting part of a pixel region, and a method for fabricating the same.
2. Discussion of the Related Art
Among various ultra-thin flat display devices with a thickness of several centimeters, an LCD device is used for various products, including notebook computers, monitors, spacecraft, and aircraft. An LCD device includes first and second substrates, and a liquid crystal layer formed between the first and second substrates. The first substrate includes a gate line, a data line, a thin film transistor, and a pixel electrode. The second substrate includes a light-shielding layer that prevents the light from leaking, a color filter layer formed on the light-shielding layer, and a common electrode formed on the color filter layer.
The LCD device may be classified into a transmitting type, a reflective type, and a transflective type. A transmitting type LCD device uses a backlight unit as a light source. However, the transmitting type LCD device consumes a large amount of power due to the use of the backlight unit. A reflective type LCD device uses the ambient light as a light source instead of a backlight unit. However, the reflective type LCD device cannot be used in dark surroundings. On the other hand, a transflective type LCD device has properties of both the transmitting type and the reflective type LCD and overcomes problems related to the high power consumption of the transmitting type LCD device and the environmental restrictions on use of the reflective type LCD device. Accordingly, a transflective type LCD overcomes these defects of the transmitting and reflective type LCD devices by including both transmitting and reflective parts in one unit pixel.
The transflective type LCD device includes a pixel electrode and a reflective plate to improve the brightness. The pixel electrode enables the light incident on the liquid crystal layer from the backlight unit to pass through the first substrate. Also, the reflective plate reflects the ambient light incident to the second substrate from the bright surroundings.
Hereinafter, a related art transflective type LCD device will be explained with reference to the accompanying drawings. FIG. 1A is a plan view of a first substrate according to a related art LCD device, and FIG. 1B is a cross section view along I-I′ of FIG. 1A.
As shown in FIGS. 1A and 1B, a first substrate includes a gate line 10, a data line 20, and a thin film transistor 40. At this time, the gate and data lines 10 and 20 are perpendicular to each other to define a pixel region. Also, the thin film transistor 40 is formed adjacent to each crossing portion of the gate and data lines 10 and 20.
On the thin film transistor 40, there is a passivation layer 70 that includes a contact hole 26 provided in a predetermined portion corresponding to each pixel region. Through the contact hole 26, a pixel electrode 30 is connected to a drain electrode 24 of the thin film transistor 40. Then, a reflective plate 35 is formed on a predetermined portion of the pixel electrode 30. While the pixel electrode 30 is formed of a transparent conductive material, the reflective plate 35 is formed of a metal material having good reflectivity. The reflective plate 35 is formed only in a reflective part, shown in FIG. 1B.
The thin film transistor 40 includes a gate electrode 12 that is connected to the gate line 10, a gate insulating layer 60 that is formed on an entire surface of the first substrate including the gate electrode 12 and the gate line 10, a semiconductor layer 19 that is formed on the gate insulating layer 60 above the gate electrode 12, a source electrode 22 that is connected to the data line 20 being positioned above one end of the semiconductor layer 19, and a drain electrode 24 that is electrically connected with the pixel electrode 30 through the contact hole 26, being positioned above the other end of the semiconductor layer 19.
The above-mentioned transflective type LCD device includes a wavelength plate 53 and a polarizing plate 56 that control light incidence and emission. The wavelength plate 53 and the polarizing plate 56 are formed on the outer surface of each of the first substrate and the second substrate (not shown) to control the incidence and emission of the ambient light. The wavelength plate 53 corresponds to a quarter wave plate (QWP) having a phase difference of λ/4. Accordingly, the wavelength plate 53 changes the polarization state of light by either changing linearly polarized light to elliptically polarized light, or by changing elliptically polarized light to linearly polarized light. The polarizing plate 56 is adhered to the outer surface of the wavelength plate 53. The polarizing plate 56 transmits only the light that is parallel to a light-transmittance axis. Accordingly, ambient light becomes linearly polarized after passing through the polarizing plate 56.
A backlight 80 is positioned beneath the polarizing plate 56. When the light emitted from the backlight 80 is incident on the first substrate by a light-guiding plate 85, the light being incident on the reflective part is completely blocked by the wavelength plate 53 of the first substrate after being reflected on the reflective plate 35. Thus, the reflected light may not be redirected to the transmitting part.
If the area of the reflective plate in the pixel region increases, the reflective brightness can be improved in a reflective mode in which the backlight is turned OFF. However, due to the small aperture ratio of the transmitting part for transmitting the light emitted from the backlight, the transmitting brightness is degraded in the transmitting mode. On the other hand, if the area of the reflective plate in the pixel region decreases, the transmitting brightness is enhanced in the transmitting mode due to the large aperture ratio of the transmitting part for transmitting the light emitted from the backlight. However, the reflective brightness is degraded in a reflective mode in which the backlight is turned OFF.
Accordingly, there is a trade-off between the operation between the reflective mode and the transmitting mode in a transflective type LCD device.